1. Field of the Invention
The present invention relates to a roundness measuring device, method and program for measuring the roundness of a measured object.
2. Description of the Related Art
Roundness measuring devices are used to measure the roundness of columnar or cylindrical workpieces (measured objects). Such roundness is measured by mounting a workpiece on a turntable (centering table), rotating the turntable (table rotation type) or revolving a stylus itself around the workpiece, and then tracing the round surface of the workpiece (such as the outer or inner surface) with the stylus. A roundness measuring device of table rotation type obtains measurements based on a displacement of its stylus on a predetermined axis. Examples of such roundness measuring devices are disclosed in Patent Documents 1 to 5 (Patent Document 1: Japanese Patent Laid-Open No. (HEI) 4-329306; Patent Document 2: Japanese Patent Laid-Open No. (HEI) 5-269649: Patent Document 3: Japanese Patent Laid-Open No. (HEI) 8-122050; Patent Document 4: Japanese Patent Laid-Open No. 2004-93529; and Patent Document 5: Japanese Patent Laid-Open No. 2004-108787).
Each of the above-mentioned measurement methods is premised on the assumption that a contact point between a stylus and a workpiece should be located on a predetermined axis (detection line) where the displacement of the stylus can be detected, so that more precise measurements may be obtained by the roundness measurement device of table rotation type. However, such a stylus has a radius with a predetermined distance from its center to the surface, of course. Accordingly, if the workpiece is eccentric with respect to the rotation axis of the centering table, some occasions may arise where the contact point is not located on a predetermined axis depending on the radius of the stylus, which would lead to errors in measurements. Such error problems would notably occur when a measurement is performed on a workpiece in an eccentric position that has a smaller radius than that of the stylus.
Therefore, an object of the present invention is to provide a roundness measuring device that can obtain a measurement with a high degree of accuracy even if a workpiece is eccentric with respect to the rotation axis, and to provide a method of and program for measuring roundness.